The present invention relates in general to aligning the frequency response of a variable bandpass filter in a radio receiver, and more specifically to obtaining a reference frequency from a digital processor clock in an alignment procedure for a variable bandpass filter.
Radio wave receivers such as AM and FM radio, TV, and mobile transcievers simultaneously receive many different broadcast signals on their antennas. A desired signal must be separated from others in order to present it to the user of a receiver.
Any desired signal occupies a unique frequency range. Thus, a desired signal may, at least partially, be separated from other signals by connecting an antenna to a bandpass filter which attenuates all frequencies other than those in the desired frequency range. In order to selectively receive different frequency ranges (i.e., stations or channels), the .bandpass filter comprises a variable radio-frequency (RF) filter. One such variable filter includes voltage variable capacitors known as varactors, as well as fixed capacitors and fixed inductors in a bandpass configuration wherein a voltage applied to the varactors controls the resonant frequency (i.e., center frequency) of the filter. When tuning in a radio wave broadcast signal at a particular carrier frequency, a control voltage applied to the varactors causes the center resonant frequency of the RF filter to coincide with that carrier frequency.
Due to variability of characteristics of individual capacitors, inductors, and varactors used in constructing the RF filters, the filter response must be aligned (i.e., calibrated by frequency) for each individual receiver.
In order to find the precise control voltage to be applied to a filter to obtain a desired frequency response, a known reference frequency is typically applied to the input of the variable filter during the alignment procedure. The filter control voltage is then varied throughout its range in order to maximize the magnitude of the filter output. At maximum output, the frequency response of the variable filter is assumed to be properly aligned.
Alignment is preferably done automatically without use of external devices in order to save manufacturing time, labor, and cost. Prior art receivers with automatic alignment have obtained a reference frequency from either a local oscillator already present in the radio receiver or an auxiliary source dedicated to providing the reference frequency.
The use of extra hardware is undesirable, especially at higher frequencies such as within the FM band, television band, and mobile communication band because of high cost. If the local oscillator present in the receiver is used to provide the reference frequency, then the local oscillator is not available to perform its usual task of shifting the radio frequency signal to a lower intermediate frequency. In that case, the magnitude of the radio frequency signal must be measured and maximized. However, detection of the level of an RF signal can only be done in an inaccurate manner subject to many errors unless expensive and complicated extra hardware is employed.